I finally got the scoop on this one..
You need to enter/exit 100 dungeons (and here's the kicker) in ONE SESSION! That is, you have to start the game and go in and out of 100 dungeons before you quit or get kicked out of the game for whatever reason. Should you leave the game the counter gets reset to 0.
There's more...
Multilevel dungeons - such as Gronkor's Crypt - count multiple times. Let's say you go inside, then go down to the 2nd level, come back up and exit - that counts as 3. Why? Because you entered the first level, entered the 2nd level, entered the first level again, then went outside.
Furthermore... There's NOTHING that says the dungeon has to be UNIQUE. Based on that, you can go to say, the bear cave (from Tutorial: Relics) or any other cave that's relatively safe to enter (where you won't get immediately jumped by something nasty) and go in and out 100 times and be done with it.
Btw.. This information is available on the Sacred Wiki here...
P.S. Welcome to DarkMatters!
Indeed and agreed
Just a lot of clicking and hoping you won't get kicked for some unknown "feature" the game just desided to let you know about
Have fun and really it doesn't take that long